1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, which forms images utilizing the electrophotographic method. More particularly, the invention relates to an image forming apparatus provided with a fixing device that thermally fixes the unfixed toner images transferred to a recording material.
2. Related Background Art
To form images, the image forming apparatus that utilizes the electrophotographic method is arranged to charge the surface of the electrophotographic photosensitive body uniformly by use of the charging device, and to expose the charged surface of the electrophotographic photosensitive body by use of the exposing device, thus forming the electrostatic latent images. Then, the electrostatic latent images are developed by use of the developing device to form toner images. The toner images are transferred to a recording material, such as a recording sheet, by use of the transferring device. The toner images are then fixed on the recording material by use of the fixing device as permanently fixed images for output.
As shown in FIG. 8, the aforesaid fixing device is, for example, arranged to be able to pinch and convey by use of the guiding member 18 a recording material P which is provided with the unfixed toner transferred to it on the fixing nip unit N between the fixing roller 100 and the pressure roller 110, and then, to heat and press the toner T by the heater 13 that serves as the heating body arranged in the fixing roller 100 in order to fix the unfixed toner (unfixed toner images) T on the recording material P as the permanently fixed images.
The fixing roller 100 is structured with the releasing layer 10b formed by PFA, PTFE, or some other fluororesin on the core metal 10a formed by aluminum, iron, or the like. In the releasing layer 10b, the heater 13 is arranged as heating means to heat the fixing roller 100 from its inside.
The releasing layer 10b is provided for the prevention of the "offset phenomenon" which tends to stain the images to follow after the transfer of the unfixed toner images to the fixing roller 100. Here, further, by the utilization of the electrostatic charge given characteristically to the toner (that is, the negative polarity in the specification hereof), the electric potential of the negative polarity is generated on the surface of the fixing roller 100 when the bias of the negative polarity is applied to the core metal 10a from the fixing bias supply source 12. Thus, by the utilization of the force of repulsion of the negatively charged toner, the toner is allowed to adhere to the surface of the fixing roller 100 so as to prevent the occurrence of the offset phenomenon that may result in defects on the images to follow.
On the fixing roller 100, there are arranged the separating claws 14, and the thermistor 15 which serves as temperature detecting sensor.
The separating claws 14 are in contact with the surface of the fixing roller 100 to prevent the recording material P from being wound around the fixing roller 100 when it passes the fixing nip unit N. It is generally practiced to use the separating claws 14 which are coated with PFA, PTFE, or some other fluororesin having an excellent releasability for the prevention of the adhesion or accumulation of toner to or on the surface of the separating claws 14.
The thermistor 15 is in contact with the surface of the fixing roller 100 under a specific pressure to detect the surface temperature of the fixing roller 100. On the basis of the information of the surface temperature of the fixing roller 100 thus detected by the thermistor 15, the current to the heater 13 is turned on and off by use of a controller (not shown) so that the surface temperature of the fixing roller 100 is kept at a constant level.
The pressure roller 110 is pressed to the fixing roller 100 by use of pressure means (not shown) to form the fixing nip unit N. The pressure roller 110 is provided with the elastic layer 11b which is formed by the heat resistive silicon sponge rubber or the like on the core metal 11a formed by aluminum, iron, or the like, and on the elastic layer 11b, there is further provided the releasing layer 11c which is formed by the PFA, PTFE, or some other fluororesin having a minute quantity of conductive particles, such as carbon, dispersed in it.
With the minute quantity of the conductive particles, such as carbon, which is dispersed in the fluororesin, the releasing layer 11c is provided with the resistive value so that it can ground the releasing discharge generated between the trailing end of the recording material P and the surface of the fixing roller 100 to the GND of the apparatus main body through the surface of the pressure roller 110 the moment the trailing end of the recording material P is parted from the fixing roller 100.
The diode element 17 is arranged to connect the core metal 11a of the pressure roller 110 and the GND of the apparatus main body. The cathode of the diode element 17 is connected with the core metal 11a, and the anode thereof is connected with the GND of the apparatus main body so as not to allow the potential of the core metal 11a to present the negative polarity. Then, the potential difference is provided between the surface of the fixing roller 100 and the core metal 11a of the pressure roller 110 so that with the electric field thereof, it is arranged to prevent the offset phenomenon from taking place between the recording material P and the fixing roller 100.
Now, as shown in FIGS. 9A to 9D, the aforesaid fixing device allows the unfixed toner or the offset toner described above to adhere to its fixing roller 100 when the image formation is carried out on a specific number of recording sheets. As a result, the toner T1 is accumulated on the surface where the separating claws 14 and the fixing roller 100 are in contact (FIG. 9A).
Then, when the toner T1 is accumulated to a certain extent, it is transferred from the separating claws 14 to the fixing roller 100, and transferred further to the pressure roller 110, hence adhering to the recording material P again as the stained toner T1 (FIGS. 9B, 9C, and 9C).
The transfer of the toner T1 from the separating claws 14 to the fixing roller 100 is a phenomenon that tends to occur more often when the fixing roller 100 begins to rotate, that is, the first run in the morning with the fixing device being in the state of the room temperature or at the actuation in the state on standby. The toner T1 transferred to the fixing roller 100 or the pressure roller 110 is again transferred to the recording material P when the recording material P passes the fixing nip unit N.
The separating claws 14 are aimed at separating the recording material P from the fixing roller 100 exactly. Therefore, it is necessary to arrange a plurality of the separating claws to be in contact with the recording material P in the longitudinal direction of the image area thereof. Also, the contact surface of the separating claws 14 on the fixing roller 100 should be provided with some measures to make the adhesion or accumulation of toner difficult. As such means, the surface layer of each separation claw 14 is coated with PFA, PTFE, or some other fluororesin having an excellent releasability.
Nevertheless, with the excellent releasability which is given to the separating claws 14 alone, it is insufficient to reduce the toner stains that may be allowed to adhere to the recording material P, although there is an effect that the adhesion of toner to or the accumulation thereof on the leading end portion of each separation claw can be reduced.
Also, this phenomenon may lead to the repetition of the cycle of the adhesion of the offset toner to or the accumulation thereof on the separating claws 14, which is peeled off to fall when the fixing roller 100 is actuated, and again its adhesion or accumulation of the offset toner, which is peeled off to fall when the fixing roller 100 is actuated.
This phenomenon does not present any problem for a sheet or two or a continuous printing of several sheets only, because the accumulated amount of toner is minute, and even if it is peeled off from the separating claws 14 to fall and adhere to the recording material P as stains through the fixing roller 100 and the pressure roller 110, the peeled off amount of toner is so small that the resultant stains on the recording material P is not recognized by eye-sight as stains.
However, if a printing is continued for several tens of sheets to several hundreds of sheets, the amount of the accumulated toner becomes greater between the separating claws 14 and the fixing roller 100, and the toner peeled off from the separating claws 14 becomes recognizable by eye-sight on the recording sheet as stains. In this way, the creation of this phenomenon becomes more conspicuous when the printing is continuously made for several tens of sheets or more.